Hybrid vehicles potentially offer fuel efficiency and vehicle driving range improvements over non-hybrid vehicles. One example of a hybrid vehicle includes an engine that may be selectively coupled to an electrical machine and a transmission according to vehicle operating conditions. The engine may be selectively coupled to the electric machine and transmission via an electrically or hydraulically actuated disconnect clutch. The disconnect clutch allows the electric machine to provide torque to vehicle wheels during low torque demand conditions without having to operate the engine and without having to supply torque to rotate an engine that is not combusting an air-fuel mixture. The disconnect clutch may also be used to restart the engine from a condition of no rotation via the electric machine.
Selectively coupling an electric machine to an engine via a disconnect clutch may also present issues that are not observed in a non-hybrid vehicle. For example, engine starting may introduce vibrations and torque impulses to the vehicle driveline. If the disconnect clutch is applied too abruptly, noise and vibration may be introduced to the vehicle driveline such that the driver may be disturbed. Likewise, if the disconnect clutch is released too abruptly, the driver may be disturbed. Finally, it may be desirable to improve disconnect clutch operation so that smooth transitions between operating the vehicle without the engine and operating the vehicle with the engine may occur.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method of operating a hybrid vehicle comprising, during selected engine shut-down conditions, releasing a disconnect clutch coupled between an engine and an electric machine in a vehicle driveline, and after disconnect clutch release, estimating torque errors based on deviations of an actual engine speed profile from a commanded engine speed profile at each of a plurality of engine speed set-points. In this way, torque errors may be learned.
In one example, during an engine shut-down condition, the engine may be selectively deactivated and a disconnect clutch coupled between the engine and the rest of the vehicle driveline, including an electric machine, a transmission, and vehicle wheels, may be released. Based on the vehicle speed and pedal position at the time of shut-down, a rolling stop may be expected. An engine controller may determine an engine torque to be commanded before the clutch is released. The commanded engine torque may be such that substantially zero torque is maintained across the disconnect clutch upon release. In the absence of engine torque estimation errors, following disconnect clutch release, the engine may follow an expected speed profile that is based on the commanded torque. Therefore, based on deviations of the actual engine speed profile from the expected value, torque over-estimation and under-estimation errors may be learned. In particular, the speed profile may be assessed in a window surrounding one or more selected engine speed set-points. The speed set-points may be selected based on engine torque error history and may correlate with speed set-points where driveline disturbances have been experienced. The errors may be learned as a function of engine speed and actuator settings. The learned errors may then be used to update a torque estimation model, and control engine torque during a subsequent engine restart when the clutch is applied.
In this way, torque estimation errors may be learned and torque control may be improved. By learning torque errors and updating a torque estimation model, driveline torque disturbances of a hybrid driveline may be reduced. Further, the approach may improve vehicle drivability. Further still, the approach may reduce driveline wear, thereby increasing the operating life of the driveline.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.